1. Field of the Invention
This invention relates to a novel resin composition, and more particularly to a radically setting resin composition which sets with moderate shrinkage and possesses excellent adhesiveness and electrical properties.
2. Description of Prior Arts
Unsaturated polyester resins are less viscous and, therefore, easier to handle than epoxy type resins and other similar resins. Moreover, they quickly set even at normal room temperature and atmospheric pressure. Further, the products they form on setting possess fine properties. The resins, therefore, are used in large quantities in various fields.
Despite such beneficial properties, the unsaturated polyester resins have a disadvantage that they substantially exhibit heavy shrinkage during the setting. This difficulty stands on the way to further expansion of their use. To remedy the difficulty, methods for mitigating the heavy shrinkage of unsaturated polyester resins by incorporation therein of thermoplastic polymers have been developed and accepted for actual use in some fields. Generally, unsaturated polyester resins and thermoplastic polymers are not fully compatible with each other. When they are mixed, the thermoplastic polymers begin to separate from the polyester resins in a relatively short period of time. These methods, therefore, pose a problem that the combination fails to produce compositions capable of retaining the constituent parts stably mixed.
In terms of adhesiveness, unsaturated polyester resins are decidedly inferior to epoxy type resins, phenol resins, etc. In the circumstances, methods aimed at improving their adhesiveness as by incorporating a dicyclopentadiene structure into their unsaturated polyester backbones or modifying unsaturated polyesters with a xylene-formaldehyde resin have been proposed. These methods, however, have fallen short of producing the effects aimed at.
Further, Japanese Laid-open Patent Publication No. 60,984/1978 discloses a resin composition comprising an unsaturated polyester resin and a dicyclopentadiene type thermoplastic polymer in order to decrease setting shrinkage and improve adhesiveness. However, it was difficult to attain higher adhesiveness which is one of the object of the present invention by such method. It is thought that the reason is based on the following reasons: Dicyclopentadiene skeleton which is deemed to contribute increasing adhesiveness is introduced as a thermoplastic polymer, so the amount is limited; and it is apt to set heterogeneously, so the skeleton are localized in the set product. Furthermore, according to the above method, it was necessary to decrease the amount of the dicyclopentadiene skeleton in the thermoplastic polymer and also decrease the molecular weight of the polymer in order to enhance the compatibility, because the method can attain low compatibility.
An object of the present invention, accordingly, is to provide a curable resin composition which sets with moderate shrinkage and possesses high adhesiveness and electrical properties.